The present invention relates to a sensor apparatus for transmitting electrical pulses from a signal line into and out of a vessel to measure a process variable. A single conductor surface wave transmission line (Goubau line) is adapted as a sensor for industrial process variable measurement, in particular for level measurement. Such devices are intended for use for example in the process and storage industry.
A pulse sent down a probe is affected by any change of the electrical properties of the surroundings of the probe. A material located inside the vessel for example causes a change in electrical impedance at the material surface. At least part of the pulse will thus be reflected at the surface. The level of the material inside the vessel can be determined from the time required for the pulse to propagate to the surface and back.
Other process variables can be determined. The amplitude of a reflected pulse for example is a measure of the change in impedance at the reflecting surface and can be used to determine the dielectric constant of the material. Also it is feasible to measure thickness and/or dielectric constants of layers of different materials stored in a vessel from the amplitude and the time-of-flight of the respective number of reflected pulses.
Recent developments by the National Laboratory System now make it possible to generate fast, low power pulses, and time their return with very inexpensive circuits. See, for example, U.S. Pat. Nos. 5,345,471 and 5,361,070 assigned to The Regent of the University of California. The pulses generated by this new technology are broadband, and also are not square wave pulses. In addition, the generated pulses have a very low power level. Such pulses are at a frequency of 100 MHz or higher and have an average power level of about 1 nano Watt or lower. These factors present new problems that must be overcome to transmit the pulse down and back and to process and interpret the returned pulses.
It is of essential importance to provide a design for the sensor apparatus which ensures a high mechanical stability suitable for industrial applications while at the same time maintaining the electrical operation of a Goubau line. This includes ensuring a smooth impedance transition of the pulse from the signal line and transmission through the mounting to the probe and vice versa. Changes in electrical impedance throughout the apparatus, i.e. the signal line, the mounting area and the probe inside and outside the mounting section are to be avoided. Every electrical impedance discontinuity causes a partial reflection of energy of the pulse and thus reduces the signal to noise ratio.
In copending U.S. patent application Ser. No. 08/574,818 entitled SENSOR APPARATUS FOR PROCESS MEASUREMENT filed on Dec. 19, 1996 and a related Continuation in Part Application U.S. Patent Application Serial No. 08/735,736 with the same title filed on Oct. 23, 1996 sensor apparati for transmitting electrical pulses from a signal line into and out of a vessel to measure a process variable are described.
A sensor apparatus is described comprising:
a mounting section configured to be coupled to the vessel, PA1 a cylindrical guide element located inside the mounting section, PA1 means for preventing a movement of the guide element in a direction toward the vessel, PA1 a conductive probe element mounted inside the mounting section and extending through the cylindrical guide element into the vessel, PA1 a metallic insert located inside the mounting section, and PA1 an electrical connector PA1 a mounting section configured to be coupled to the vessel, PA1 a cylindrical guide element located inside the mounting section, PA1 means for preventing a movement of the guide element in a direction toward the vessel, PA1 a conductive probe element mounted inside the mounting section and extending through the cylindrical guide element into the vessel PA1 a metallic insert located inside the mounting section, PA1 an electrical connector
coupled to the probe element, PA2 configured to couple the signal line to the probe element. PA2 pressed into the mounting section in a direction towards the vessel by a spring element and exerting a force on the guide element in that direction, and PA2 fastened to the metallic insert and PA2 configured to couple the signal line directly to the probe element.
The insert comprises a thread and is screwed into the mounting section in a direction towards the vessel. It prevents a movement of the guide element in a direction away from the vessel. It is also described, that the insert alternately may be snapped in and held with a spring element and a retaining ring. The insert includes an air filled conical cavity. A conical steel nut, serving as an impedance transitioning element and as an intermediate connecting element, is located within the cavity and a high frequency electrical connector is coupled to an end of the nut facing away from the vessel via a pin with decreased diameter. This pin is slidable within an aperture of the cone to permit some movement of the probe element. Such a movement may occur for example when pulling forces act on the probe. This design is suitable for industrial applications, e.g. for measuring the level of a liquid in a vessel, where pulling forces are fairly constant or vary slowly with time. In applications where the probe is subjected to abrupt changes of the pulling forces, e.g. for measuring the level of solids, this design shows some disadvantages. If the position of the conical steel nut with respect to the insert is altered, the impedance matching essential for a smooth impedance transition from the signal line to the probe is impaired. Also a movement of the pin within the connector may change the electric properties of the connector and thus affect the quality of the signal transition.
It is an object of the invention to provide a sensor apparatus which can be used in industrial applications, which has a high mechanical stability, tight pressure seals and which can withstand high pulling forces while at the same time maintaining effective level, distribution, transition and control of high frequency impedances throughout the sensor apparatus.
To this end the invention comprises a sensor apparatus for transmitting electrical pulses from a signal line into and out of a vessel to measure a process variable, the sensor apparatus comprising:
According to a refinement of the invention, the axial length of the guide element is large compared to an outer diameter of the probe element.
According to a refinement of the invention the means for preventing a movement of the guide element in a direction toward the vessel is an outwardly tapered outer surface of the guide element abutting on an outwardly tapered inner surface of the mounting section.
According to a refinement of the invention the probe element includes a head portion comprising an outwardly tapered surface facing towards the vessel and engaging an outwardly tapered inner surface of the guide element located adjacent to a surface of the guide element facing away from the vessel.
According to a refinement of the invention a dielectric insert is located inside the mounting section between the guide element and the metallic insert.
According to a refinement of the invention the probe element includes a head portion comprising an inwardly tapered surface facing away from the vessel and engaging an inwardly tapered surface of the dielectric insert.
According to a refinement of the invention a recess is located within the head portion of the probe element forming a backing support for a male pin of the electrical connector.
According to a refinement of the invention the probe element comprises a hollow cylinder extending in a direction away from the vessel toward the connector and the male pin of the connector extends through the hollow cylinder.
According to a refinement of the invention the probe element has an elongated section extending into the vessel.
According to a refinement of the invention the elongated section comprises a hollow section facing toward the vessel and wherein one end of a probe extension, in particular a rod or a wire, is mounted inside the hollow section.
According to a refinement of the invention the metallic insert is electrically connected to ground potential.
According to a refinement of the invention the guide element is made of a dielectric material.
According to a refinement of the invention a first seal, in particular an o-ring, is provided between the mounting section and the dielectric insert and a second seal, in particular an o-ring, is provided between the probe element and the dielectric insert.
The invention and its advantages are explained in more detail using the figures of the drawing, in which an exemplary embodiment is shown. The same reference numerals refer to the same elements throughout the figures.